1. Field of The Invention
The present invention generally relates to a ridge waveguide type photo semiconductor device and a method for fabricating the same.
2. Description of Related Art
In recent years, the demand for Internet and data communication are considerably increasing. In accordance therewith, the capacities of optical communication systems are rapidly increasing. The communication capacities of trunk systems reach 10 Gb/s per channel, and multichannel transmission systems based on the wavelength division multiplex system are intended to be introduced, so that it is desired that rapidly modulatable photo semiconductor devices are provided at low costs.
Ridge waveguide type photo semiconductor devices (light modulators, semiconductor lasers, etc.) do not have any semiconductor buried layers which are provided in most of refractive index waveguide structures and which function as current blocking layers. Therefore, the ridge waveguide type photo semiconductor devices have an advantage in that current leakage and reverse junction breakdown do not occur in buried layers, and an advantage in that the devices do not have parasitic capacities caused by buried layer junctions, so that the parasitic capacities of the whole devices can be decreased. Moreover, processes for fabricating ridge waveguide type photo semiconductor devices have excellent cost reducing effects and mass production effects since only one or two crystal growths have only to be carried out. Thus, ridge waveguide type photo semiconductor devices have excellent advantages. On the other hand, there is a problem in that it is difficult to carry out processes for fabricating ridge waveguide type photo semiconductor devices. As a typical example of a ridge waveguide type photo semiconductor device, a conventional device structure of a ridge waveguide type semiconductor laser (which will be also hereinafter referred to as a RWG-LD) will be described below.
FIG. 12 shows the structure of a conventional Fabry-Perot type RWG-LD device. On a substrate 80 of n-InP, a cladding layer 81 of n-InP in which S is doped and which has a thickness of 2 μm and a carrier density of 1×1018 cm−3, an active layer 82 which has a thickness of 1.3 μm and a multiple quantum well structure (which will be also hereinafter referred to as MQW), a cladding layer 83 of p-InP in which Zn is doped and which has a thickness of 2 μm and a carrier density of 1×1018 cm−3, and a contact layer 84 of p-InGaAs in which Zn is doped and which has a thickness of 0.3 μm and a carrier density 8×1018 cm−3, are sequentially crystal-grown by the MOCVD (Metal Organic Chemical Vapor Deposition) method. Subsequently, after an SiO2 film (not shown) is formed on the whole surface of the wafer, a resist pattern (not shown) having a width of 5 μm and two stripe-like windows is formed on the wafer by the lithography technique, and the resist pattern is used as a mask for etching the SiO2 film so that the film has a stripe shape. After the resist pattern is removed, the SiO2 film is used as a mask for sequentially etching the p-InGaAs contact layer 84 and the p-InP cladding layer 83 with a sulfuric acid containing etchant and a hydrogen bromide containing etchant, respectively. At this time, the etching is stopped directly above the MQW active layer 82, and a reverse mesa-shaped ridge 86 is formed. Thereafter, the SiO2 film is removed.
Then, after an SiO2 film 85 is formed on the whole surface of the wafer, a resin 87 is applied thereon and cured. Thereafter, the head of the ridge portion 86 is exposed by the reactive ion etching (which will be also hereinafter referred to as RIE). By this operation, the resin 87 can be filled in grooves on the both sides of the ridge 86. Subsequently, a resist pattern (not shown) having a stripe-like window having a width of 2 μm is formed on the ridge 86.
Then, after a TiPt film 88 to be a p-type electrode is deposited on the whole surface of the wafer, a stripe-like TiPt film 88 is formed on a portion of the ridge 86 corresponding to the window by the lift-off method. After sintering is carried out at 450° C., a bonding pad 89 of Ti /Pt/Au is formed by the vapor deposition method and the lift-off method.
Then, the n-InP substrate 80 is polished so as to have a thickness of 100 μm, and an AuGe/Ni/Au film 90 to be an n-type electrode is deposited on the reverse surface of the wafer to be sintered. The wafer is cut out so as to have a resonator length of 300 μm and a chip width of 300 μm, so that a device is completed.
FIG. 13 shows an equivalent circuit of an RWC-LD thus constructed. In FIG. 13, a current is injected into the active layer 93 via a bulk resistance 92 of the ridge-shaped p-InP cladding layer 91, and a current also flows through a diffusion resistance 94 in lateral directions. The current flowing through the diffusion resistance 94 becomes a reactive current (leakage current), and does not contribute to laser oscillation. In addition, a parasitic capacity 95 of a pn junction exists in lateral directions, so that there is a problem in that the capacity of the device increases if the diffusion resistance 94 is low.
FIG. 14 shows the construction of another conventional ridge waveguide type semiconductor laser. FIG. 14 is a sectional view perpendicular to optical waveguide directions. In FIG. 14, reference number 151 denotes an n-type InP substrate, 152 denoting an n-type InP cladding layer in which S is doped and which has a thickness of 2.0 μm and a carrier density of 1×1018 cm−3, 153 denoting an active layer having a multiple quantum well structure of InGaAsP/InGaAsP, 154 denoting a p-type InP cladding layer in which Zn is doped and which has a thickness of 0.2 μm and a carrier density of 5×1017 cm−3, 155 denoting a p-type InGaAsP etch stop layer in which Zn is doped and which has a band gap wavelength of 1.2 μm, a thickness of 0.02 μm and a carrier density of 1×1018 cm−3, 156 denoting a p-type InP over cladding layer in which Zn is doped and which has a thickness of 1.5 μm and a carrier density of 1×1018 cm−3, 157 denoting a p-type InGaAs contact layer in which Z is doped and which has a thickness of 0.3 μm and a carrier density of 8×1018 cm−3, 158 denoting an SiO2 film, 159 denoting a resin, 160 denoting a p-side ohmic electrode of Pt/Ti/Pt, 161 denoting a wire/bonding pad of Ti/Pt/Au, and 162 denoting an n-side ohmic electrode of AuGe/Ni/Au.
In this conventional example, since the p-type InP cladding layer 154 is formed on the whole top face of the active layer 153, a current injected from the p-type InP over cladding layer 156 diffuses to the outside of a ridge portion 163 via the p-type InP cladding layer 154. As a result, the p-type InP overlap cladding layer 156 picks up the junction capacity outside of the ridge portion 163 although it is formed so as to have a ridge shape, so that it is difficult to reduce the capacity of the device.
FIG. 15 is a sectional view of another ridge waveguide type semiconductor laser. In this figure, the same reference numbers as those in FIG. 14 are given to the same portions as those in FIG. 14, and the descriptions thereof are omitted. In this conventions example, only the ridge-shaped p-type over cladding layer 156 is stacked on the active layer 153, and the diffusion of current to the outside of the ridge portion 163 hardly occurs. Therefore, it is possible to sufficiently reduce the capacity of the device, and it is possible to carry out a rapid modulation of 10 Gb/s. However, there is a problem in that the light confining effect to the active layer is inferior to that of the ridge waveguide type semiconductor laser shown in FIG. 14.
Ridge waveguide structures obtain the light confining effect in vertical directions by sandwiching a light waveguide layer (the active layer 153 in the conventional example shown in FIG. 15) between cladding layers having a low refractive index (the n-type InP cladding layer 152 and p-type InP over cladding layer 156 in this conventional example). On the other hand, the light confining effect in lateral directions is realized by forming an one-side ridge-shaped cladding layer (the p-type InP over cladding layer 156 in the conventional example shown in FIG. 15). In the conventional example shown in FIG. 15, only the n-type InP cladding layer 152 exists on one side of the active layer 153 in regions other than the ridge portion, and the opposite side directly contacts the SiO2 film 158. Since the dielectric of the SiO2 film or the like has a far lower refractive index than that of semiconductors, the photoelectric field distribution in the active layer 153 rapidly attenuates on the side of the SiO2 film 158, so that the light confining effect to the active layer 153 in vertical directions is low.
In the conventional example shown in FIG. 14, the thin p-type InP cladding layer 154 is formed on the whole top face of the active layer 153 in order to obtain the light confining effect in vertical directions. The thickness of the p-type InP cladding layer 154 is set to be 0.2 μm so as not to prevent the light confining effect in lateral directions and so as to ensure the light confining effect in vertical directions.
Thus, in order to realize both of the reduction of capacity of the device and the light confinement to the active layer, it is required to further devise the structure of the cross section. In FIG. 14, if the carrier density of the p-type InP cladding layer 154 is decreased or if the p-type InP cladding layer 154 is an undoped layer, it is possible to suppress the diffusion of current to the outside of the ridge portion 163 while holding the light confining effect. However, this can not be an actual solution since the current injection efficiency to the active layer 153 immediately below the ridge portion 163 is also deteriorated.
On the other hand, as a conventional structure realizing both of the reduction of capacity of the device and the light confinement to the active layer, there is a ridge waveguide type semiconductor laser shown in FIG. 16. In this figure, the same reference numbers as those in FIG. 14 are given to the same portions as those in FIG. 14, and the descriptions thereof are omitted. In this conventional example, a p-type InP cladding layer 154 is formed on both sides of the p-type over cladding layer 156 so as to be slightly wider. The width of the p-type InP cladding layer 154 may be formed so as to be wide to an extent that it is required to ensure the light confining effect. In addition, the junction capacity contributing to the capacity of the device is also defined by the width of the p-type cladding layer 154, and can be reduced.
Referring to FIGS. 16(a) through 16(e), a process for fabricating a ridge waveguide type semiconductor laser shown in FIG. 16(f) will be described below. First, on a n-type InP substrate 151, an n-type InP cladding layer (thickness 2.0 μm, S dope, 1×1018 cm−3), an active layer 153 having an InGaAsP/InGaAsP multiple quantum well structure, a p-type InP cladding 154 (thickness 0.2 μm, Zn dope, carrier density 5×1017 cm−3), a p-type InGaAsP etch stop layer 155 (band gap wavelength 1.2 μm, thickness 0.02 μm, Zn dope, carrier density 1×1018 cm−3), a p-type InP over cladding layer 156 (thickness 1.5 μm, Zn dope, carrier density 1×1018 cm−3), and a p-type InGaAs contact layer 157 (thickness 0.3 μm, Zn dope, carrier density 8×1018 cm−3) are sequentially stacked (see FIG. 16(a)).
Thereafter, a stripe-like SiO2 mask 165 is formed on the p-type InGaAs contact layer 157. This is used as a mask for etching the p-type InGaAs contact layer 157 with a sulfuric acid containing etchant (see FIG. 16(b)). Then, the p-type InGaAs contact layer 157 is used as a mask for etching the p-type InP over cladding layer 156 with a hydrochloric acid containing etchant (see FIG. 16(c)). Moreover, the p-type InP over cladding layer 156 is used as a mask for etching the p-type InGaAsP etch stop layer 155 with a sulfuric acid etchant (see FIG. 16(d)). At this time, the p-type InGaAs contact layer 157 is simultaneously side-etched. However, since the etching rate of the p-type InGaAs contact layer 157 is higher than that of the p-type InGaAs etch stop layer 155, the width of the p-type InGaAs contact layer 157 after the etching is smaller than that of the p-type InGaAsP etch stop layer 155.
Then, the p-type InGaAs contact layer 157 is used as a mask for etching the p-type InP over cladding layer 156 with a hydrochloric acid containing etchant, and simultaneously, the p-type InGaAs etch stop layer 155 is used as a mask for etching the p-type InP cladding layer 154 with a hydrochloric acid containing etchant (see FIG. 16(e)). Subsequently, after the SiO2mask 165 is etched, an SiO2film is formed on the whole surface of the device. Thereafter, a resin 159 is filled in grooves 164 on both sides of the ridge portion 163 to be flattened. Moreover, after a p-side ohmic electrode 160 of Pt/Ti/Pt is formed on the p-type InGaAs contact layer 157, a wire/bonding pad 161 of Ti/Pt/Au is formed. Finally, an n-side ohmic electrode 162 of AuGe/Ni/Au is formed on the reverse surface of the n-type InP substrate 151, so that the ridge waveguide type semiconductor laser of FIG. 16(f) is completed.
As described above, in the fabricating process shown in FIG. 16, the device is formed by alternately repeating the selective etching of the InP layer and the selective etching of the InGaAs layer and InGaAsP layer. At the step of using the p-type InGaAs layer contact layer 157 and the p-type InGaAsP etch stop layer 155 as masks for etching the p-type InP over cladding layer 156 and the p-type InP cladding layer 154, no side etching occurs. However, at the step of etching the p-type InGaAs layer contact layer 157 and the p-type InGaAsP etch stop layer 155 with the sulfuric acid containing etchant, the side etching occurs. In addition, the p-type InP over cladding layer 156 and the p-type InP cladding layer 154 are formed so as to have different widths by utilizing the difference between the etching rates of both layers. As a result, necessarily, the process considerably lacks the controllability of width and repeatability. The amount of side etching does not only have a large wafer inplane distribution, but it is also often asymmetric with respect to the ridge portion 163. In such cases, the p-type InP over cladding layer 156 is formed so that its position with respect to the p-type InP cladding layer 154 is shifted.
Since the photoelectric field is distributed so as to extend outside of the ridge portion 163 by about 1 μm on each side, if only the width of the p-type InP cladding layer 154 is increased by about 1 μm to each of the right and left of the p-type InP over cladding layer 156, the light confining effect in vertical directions is sufficiently ensured. Even if the width of the p-type InP cladding layer 154 is further increased, the capacity of the device only uselessly increases.
Thus, in the conventional ridge waveguide type semiconductor laser fabricating method utilizing the side etching, it is very difficult to obtain a desired ridge shape with a required precision.